Common Effluent Treatment Plants Chapter 22 PDF

Chapter 22 COMMON EFFLUENT TREATMENT PLANTS The concept of common effluent treatment plant (CETP) has been accepted as a solution for collecting, conveying, treating and disposing of effluents from industrial estates. The effluents include industrial waste waters and domestic sewage generated in...

Chapter 22 COMMON EFFLUENT TREATMENT PLANTS The concept of common effluent treatment plant (CETP) has been accepted as a solution for collecting, conveying, treating and disposing of effluents from industrial estates. The effluents include industrial waste waters and domestic sewage generated in these estates. The CETPs are designed to help small scale and medium scale industries located within the estates to dispose of their effluents, which, in many cases, are small in volume but have a high pollution potential. These industries cannot afford to treat their waste waters individually because of space constraints and financial limitations. But they are expected to give at least preliminary treatment in order to ensure that the common collecting sewer remains free flowing. It may be necessary in certain cases to provide pH correction also before discharging the effluents into the collecting sewer. As is the case with any waste water treatment facility, a CETP is designed on the basis of: (a) the quality and flowrate of the waste water, (b) standard of quality of treated effluent stipulated by the pollution control authority, (c) possibility of recycle, reuse or recovery of treated waste water, (d) availability of land, labour, power for operating and maintaining the CETP, and (e) willingness of the industries located in the industrial estate to contribute towards the capital and operating expenses of the CETP. A CETP can be changed to a combined effluent treatment plant when it collects domestic sewage from surrounding areas and treats it with industrial wastes. The advantages of such a system are as follows: 1. Dilution is offered to the toxic constituents and inorganic dissolved solids in the industrial wastes. 276  COMMON EFFLUENT TREATMENT PLANTS 277 2. Continuous seeding of microorganisms is possible when the mixture is subjected to biological treatment. 3. Better control over treatment plant is possible than if sewage is treated in a separate plant. 4. Sewage provides nitrogen and phosphorus as nutrients to the microorganisms which treat industrial wastes deficient in these elements. This reduces running cost of adding chemicals to some extent. CETPs are broadly grouped under two headings, viz. (i) homogeneous, in which industries producing similar goods are located in one area, for instance, tanneries or engineering goods, or pulp and paper, etc. and (ii) heterogeneous, in which industries producing widely divergent goods are placed together, such as chemical, dairy, food and fruit processing, tanneries, pulp and paper, pharmaceuticals, etc. Designing a treatment plant for the first group is easier than for the second group, as the characteristics of raw wastes produced in the former tend to show lesser variations than the latter. Advantages of providing a CETP are the following: 1. Small and medium scale industries are relieved of the problem of treating their effluents. 2. Waste water treatment is assured, thereby helping pollution control. 3. The pollution controlling authorities have to monitor only one outlet to check the performance of the CETP. 4. Participating industries have a commitment to generate only waste waters acceptable for treatment in the CETP. 5. Industries can concentrate on finding ways and means of reducing pollution at source, reduce water and power consumption to the extent possible. Many factors need careful consideration in establishing a CETP. 22.1 OWNERSHIP Ownership can be either in (i) private sector, (ii) public sector, or (iii) joint private and public sectors. Private sector ownership: In this, individual industries form a co-operative company and become its shareholders. Advantages of this form are: 1. Expression of commitment to pollution control by the industries, 2. Short-term problems of manpower and equipment can be overcome by resources available in member industries. This form of ownership is generally accepted, although there can be conflict among member industries regarding waste water quality and quantity. All participating industries pay a fair share in the cost 278 INDUSTRIAL WASTE WATER TREATMENT of treatment, depending on quality and quantity of waste water. Failure to pay can result in imposing penalty on the defaulting industry by way of non-acceptance of its effluent into the CETP. Public sector ownership: The State Industrial Development Corporations (SIDC) provide all infrastructural facilities including CETPs. But these being government organizations, conflict of interest in regard to compliance and strict enforcement by the State Pollution Control Boards may be difficult. Joint private and public sector ownership: A company is formed by the SIDC and the Industries’ Association. This arrangement improves availability of funds, but it can result in delays in defining responsibilities. 22.2 TECHNICAL ASPECTS Technical aspects include: 1. Basic information. Information about individual units, their raw materials, products, treatment, if any is being given, quality and quantity of waste water generated, scope for reducing water consumption and waste minimization. 2. Collection and conveyance. Waste water is conveyed either by underground drainage system, or by tankers, handcarts, etc., if the volume involved is small. 3. Pretreatment. Rational pretreatment to be given by individual units irrespective of the mode of conveyance of the waste water. 4. Planning. Master plan and phasewise approach if uncertainty exists about quantity, quality and treatability. 5. Disposal. Handling, treatment and disposal of sludge generated either during pretreatment or treatment in the CETP. 22.3 FINANCIAL ASPECTS It is a common observation that the unit cost of treatment becomes very high if the waste water flow is small. Similarly, a group of heterogeneous industries will produce an effluent with varying quality; hence it becomes costlier to treat than if the group consists of homogenous industries. Topographic conditions also play an important part in arriving at the cost of treatment as well as the method of collecting and conveying waste water from member industries. After a detailed study of 40 CETPs, Kantawala et al. made the following recommendations: 1. A detailed techno-economic study of conveyance of waste waters is necessary, as it accounts for a large part of capital cost.  COMMON EFFLUENT TREATMENT PLANTS 279 2. Optimization of pretreatment by individual industries is essential. 3. Phasewise development of a CETP should be seriously considered, but the quality of the treated effluent must meet the standards laid down by the pollution control authorities. 4. Upgradation of the CETP should also be kept in mind in view of the possibility of increased flows, stronger wastes, stricter standards of treated effluent, etc. 5. In-plant pollution control should be encouraged by the Central and State Pollution Control Boards and Ministry of Environment and Forests. 6. Common effluent treatment plants should be converted into combined effluent treatment plants with obvious advantages as mentioned earlier. The status of CETPs studied is presented in Table 22.1. Table 22.1 Status of CETPs (upto April 1999) State Commissioned Completed In Statewise progress total Andhra Pradesh 3 – – 3 Gujarat 5 – – 5 Haryana 1 – – 1 Karnataka 2 1 – 3 Madhya Pradesh 1 – 2 3 Maharashtra 3 – 3 6 Rajasthan 2 – – 2 Tamil Nadu 9 8 13 30 Uttar Pradesh 2 – – 2 Delhi – – 15 15 Total 28 9 33 70 Deshpande and Sen have suggested, besides the usual factors to be considered in the design of a CETP, the classification of wastes as: (a) those amenable to biological treatment; (b) those which can be biologically treated after acclimatization, (c) those which need pretreatment, and (d) those which contain non-biodegradable and toxic pollutants. They also suggest the three following approaches for the process design of CETP: 1. Modular approach, which takes into account changes in the volume of waste water as the industrial estate develops. 2. Upgradability approach, wherein the present form of treatment would need upgradation on account of increased flows, stronger wastes, stringent effluent standards and increased possibility of recovery of valuable material from the waste water. 3. Service centre approach, which should provide a workshop, garage, library, instrumentation facility and a conference hall. 280 INDUSTRIAL WASTE WATER TREATMENT The CETP at Sarigam, under Gujarat Industrial Development Corporation (GIDC) was studied by these authors. It serves four zones, divided according to activities, viz. zone 1—mechanical and water intensive; zone 2—chemical; zone 3—housing and plastics; and zone 4— mechanical. Total flow 14,200 m3/day. Treated effluent was proposed to be reused in the estate, used for horticulture, or given to surrounding farmland during dry season. Prabhu presented a case study of a CETP whose development was done in phases. In phase 1, an equalization- cum neutralization (ECN) tank and aerated lagoon was provided. In phase 2, chemical coagulation, settling and sludge handling was introduced between ECN tank and aerated lagoon. In phase 3, an additional intermediate clarifier, second stage aeration tank and secondary clarifier were added. Sewage from nearby residential colony was added at this stage to the industrial waste to dilute high total dissolved solids (TDS) in the waste water and to seed the mixture with microorganisms. In phase 4, dissolved air flotation (DAF) was added downstream of clariflocculator. In addition, sludge thickening and centrifugation were provided. The treated effluent was discharged to the municipal sewer. A heterogeneous group of industries, numbering 348, including units doing rolling and pickling, textile processing, plating and anodizing, soap making, rubber, plastic goods and candle making was established at Wazirpur industrial area by Delhi Development Authority. The need for a CETP was felt, because the effluents damaged the sewerage system, and air pollution due to the industries had an adverse effect on the health of the workers. Characteristics of the combined waste water were as shown in Table 22.2. Table 22.2 Combined Waste Water Quality Parameter Concentration pH 3.39–6.85 Turbidity, NTU 65–600 Total suspended solids 404–1573 BOD 72–112 COD 248–900 Nickel 6.02–33.66 Total chromium 6.5–27.33 Iron 117.90–188.63 Cadmium 0.02–0.056 Zinc 1.925–2.5 Copper 1.2–2.45 Cyanide 0.6–0.9 Note: All values except pH are in mg/l.  COMMON EFFLUENT TREATMENT PLANTS 281 The waste water was acidic in nature and contained heavy metals. The treatment proposed was, therefore, equalization, neutralization with lime and settling in a clariflocculator. The settled sludge would be stored in a lagoon, while the clarified effluent would be discharged into municipal sewer. Construction, operation and maintenance of the CETP would be the responsibility of Municipal Corporation of Delhi, while the cost of operation and maintenance would be met by the industries. Another heterogeneous group of industries in a chemical complex was described by Rajagopalan et al.. The industrial complex consisted of a fertilizer unit, a calcium carbide (CaC2) unit, caustic soda plant, a PVC unit, a cement plant, thermal power station, a water treatment plant and a demineralization plant. Each of these units had its own waste water treatment plant. The authors have described them briefly and have suggested steps to improve their performance. Rao, Venkayya and Rambabu have described the problems of managing a heterogeneous CETP with bulk drug manufacture as the main contributor of waste water (65%), followed by dye intermediate manufacture (20%) and the rest (15%) miscellaneous products. The CETP received slug flows of waste water, mainly because of batch production by the drug manufacturers. There were wide variations in pH (1.0–10.0), TDS (20–350 gm/l) of which 70% were inorganic, COD (3–95 gm/l). The BOD:COD ratio varied from 0.1 to 0.8. It was, therefore, necessary to segregate high TDS effluents and give them pretreatment. Inorganic TDS in biological treatment could not be tolerated beyond 2.5%. Anaerobic treatment was totally unsuccessful. Even aerobic treatment was possible only after segregating high TDS streams. Since bulk drug making was the main contributor, a screening test to establish toxic/inhibitory nature of a waste stream was devised, based on the oxygen uptake rate. It was interesting to see that Gentamycin, a bulk drug, increased oxygen uptake rate, while Paracetamol decreased it. This was used as a basis for the screening test. It was also proposed to add about 300 m3/day of domestic sewage from surrounding areas to the total effluent in order to dilute the TDS and provide continuous seeding of microorganisms. The CETP had a treatment plant consisting of equalization-cum-neutralization, flash mixing, clariflocculation, aerated lagoon, intermediate clarifier and activated sludge process. The treated effluent was discharged into a sewer. The authors suggested improvements to the existing plant, viz. increasing the holding capacity of the ECN tank to five days and converting aerated lagoon to activated sludge process. The treated effluent could then be connected to the sewerage system. The sludge generated would be incinerated using auxiliary fuel. Rajamani et al. have described a homogeneous CETP for a cluster of 14 tanneries in Bangalore. It consists of a 150 m3 holding tank, grit chamber, an equalization tank of 100 m3 capacity, with floating aerators for mixing lime, primary clarifier, acid dosing, activated sludge process 282 INDUSTRIAL WASTE WATER TREATMENT with nutrient supplementation, floating aerators for aeration and mixing, and sludge drying beds. Daily flow treated was 1000 m3. The treated effluent was discharged into Bangalore Water Supply and Sewerage Board’s (BWSSB) drainage system, carrying a sewage flow of more than 20,000 m3/day. In order to minimize the problem of sludge disposal, it was proposed to provide chromium recovery and reuse the plant in the tanneries practising chrome tanning. REFERENCES 1. Kantawala, D.C., S.M. Prabhu and S. Shah (2000): CETPs: the unfinished agenda of wastewater treatment in small and medium scale industries in India, Paper presented at the ENVIROVISION 2000-Industrial Wastewater Recycle and Reuse, Indian Environmental Association, Mumbai. 2. Deshpande, A. and R.N. Sen (1995): Common Effluent Treatment Plant—a techno-economic solution for wastewater treatment problems of industrial estates, Paper presented at the 3rd International Conference on Appropriate Waste Management Technologies for Developing Countries. NEERI, Nagpur, February, p. 915. 3. Prabhu, S.M. (2000): Common effluent treatment plant (CETP)—a case study, Paper presented at the National Seminar on Industrial Wastes Management, Indian Water Works Association, Pune Centre and Agharkar Research Institute, Pune, August. 4. Subramanian, V. (1985): Planning and development of combined effluent treatment facility for small industries—a case study, IAWPC Annual, XII: p.18. 5. Rajagopalan, S., R.V. Kadam, M.S. Sayed, M.V. Vyas, and S.N. Puntambekar (1988): Water pollution control in a chemical complex—a case study, IAWPC Annual, XV: 42. 6. Rao, K.L. S. Venkayya and G. Rambabu (1995): Management of CETP handling heterogeneous wastes, Paper presented at The 3rd International Conference on Appropriate Waste Management Technologies for Developing Countries, NEERI, Nagpur, February, p. 893. 7. Rajamani, S., R. Suthanthrajan, E. Ravindranath, and K.V. Raghavan (1995): Combined effluent treatment system for a cluster of tanneries in Bangalore—an appropriate integrated approach, 3rd International Conference on Appropriate Waste Management Technologies for Developing Countries, NEERI, Nagpur, February, p. 909.